This invention relates generally to frequency measuring apparatus and more particularly to instantaneous frequency measurement (IFM) devices which are adapted for use in measuring the frequency of radio frequency signals.
As is known in the art, instantaneous frequency measurement (IFM) devices are used in a variety of applications, as where it is desired to measure the frequency of individual pulsed or continuous wave (cw) radio frequency signals. One type of IFM device includes a wide bandwidth radio frequency limiter circuit fed by the radio frequency input signals where the IFM device is to operate over a dynamic range of 50 db or greater. The limited radio frequency signals are then fed to at least one delay line discriminator to provide an unambiguous measurement of the frequency of the input signals. Each delay line discriminator generally includes a power divide for separating the limited radio frequency signals into two quadrature channels, the signal in one channel being delayed in phase with respect to the other channel, such phase delay being related to the frequency of the input signals. The outputs of the two quadrature channels are combined and detected to form two baseband frequency output signals which have amplitudes which are proportional to the sine and cosine of the phase difference between the signals at the outputs of the quadrature channels and hence related to the frequency of the input signals. Thus, to determine the sine and cosine functions, conventional sine/cosine microwave discriminators are used together with radio frequency limiters and gray-level encoders. While such techniques may be useful in some applications, the use of RF limiters are relatively expensive and the encoders are limited in dynamic range.